1. Field of the Invention
This invention relates to electrical power measuring devices.
More particularly the invention relates to such devices of the kind comprising: input means for producing first and second analogue signals respectively representative of the instantaneous values of a voltage and current supplied to a load; multiplier means for producing in response to said first and second signals a third analogue signal representative of the produce of said first and second signals; and converter means for producing in response to said third signal an output pulse signal having a pulse rate representative of the product of said voltage and current. Such a device is hereinafter referred to as a device of the kind specified.
It will be appreciated that by totalling the output pulses of such a device a measure of the energy consumed in the load may be obtained.
2. Description of Related Art
One feature of the design of known devices of the kind specified in the compensation of offsets in the output of the multiplier means.
To compensate for such offsets it has been proposed that the polarity of one of the first and second signals be periodically reversed at the input of the multiplier means, with a corresponding reversal in the device subsequent to the output of the multiplier means. Such an arrangement is described, for example, in U.S. Patent Specification No. 4,500.973.
In known applications of this technique it is found that where the product of the first and secondn signals is small, i.e. the level of power consumption in the load is small, the pulse rae in the output signal becomes iregular. As a result calibration of the device becomes a slow process since at low power levels it is necessary to wait for a large number of outputs pulses to establish their average rate.